, LIEUT. -COL. A. R. CLARKE OK STANDARDS OF LENGTH. 46*3 
Hence we have immediately the value of 2M — T — [_df] at the temperature observed. 
Ten comparisons were made on the 4th, 5th, and 6tli of November, 1869, at temperatures 
between 50 -73 and 49°-83. The mean of the observations gave 
2M-T-[<Z/] -277-10 + 0-25 
at the temperature of 50°-33. Now, referring to pages 144, 69, and 110 of the ‘Com- 
parisons of Standards,’ we find that at the temperature of 50°-33 
2M— T— [d/]=276-12±0-38. 
The agreement of results so totally independent of one another, and each obtained 
through a very complicated system of observations, must be considered very satisfactory. 
Comparisons of the American Metre with the Ordnance Survey Metre. 
In the Appendix No. 26 of the United States Coast Survey Report for 1862 will be 
found, in connexion with the experiments for determining the lengths of the six-metre 
standard bar and its rate of expansion, mention of six iron metres designated No. 1, 
No. 2, . . . No. 6, which were compared with the “ Committee Metre,” or supposed 
standard metre. These bars are all end measures; the section is a rectangle of 1-13 
inch by 0-27 inch. 
The unit of length to which all distances measured in the Coast Survey are referred 
is the French metre , an authentic copy of which is preserved in the archives of the Coast 
Survey Office. It is the property of the American Philosophical Society, to whom it 
was presented by Mr. Hassler, who had received it from Tralles, a member of the 
French Committee charged with the construction of the standard weights and measures 
according to the decimal system. This metre is of iron, and was one of the twelve 
original bars used in the construction of the standard metre by comparison with the 
toise , which had served as unit of length in the measurement of the meridional arcs in 
France and Peru. It possesses all the authenticity of any original metre extant, bearing 
not only the stamp of the Committee, but also the original mark ( . ; ) by which it was 
distinguished from the other bars during the operation of standarding. It is always 
designated as the Committee metre. 
The iron metres have in all comparisons been placed on edge, and supported at one 
fourth and three fourths of their length. The metre compared at Southampton was 
that marked No. 6. 
The length of this metre increased by the two parts of the contact-apparatus was 
compared with the Ordnance metre. Twenty-eight comparisons were made in January 
and February 1868, eight in October of the same year, and ten in May 1869. Reducing 
the observations by least squares, we find the probable error of a single comparison 
±0-771; 
and finally, both bars being at 32°, M„ representing the American metre, 
M a =M o +44-37±0-33, 
3 q 2 
